G50 PROCEEDINGS OP THE NATIONAL MUSEUM. vol. 40. 



and, once we grasp the idea that Marsupites may be a comatulid, it 

 is readily seen that, with due regard for contingent circumstances 

 consequent on a curiously specialized mode of existence, the remain- 

 ing structures point to the same conclusion. 



When any member of a group of animals adopts a mode of life 

 entirely different from that of all the other members of the same 

 group we must be prepared to encounter extraordinary and unex- 

 pected changes in its organization which are not connected with the 

 more normal type of organization by any intermediates; and it must 

 be remembered that such changes" affect first of all the general body 

 form. Among such animals we almost always find the group charac- 

 ters developed in a most erratic manner; some structures will be 

 very highly specialized, sometimes specialized far beyond what is 

 seen in any other members of the group, while others will be in a 

 very rudimentary or primitive state of development, or even absent 

 altogether. 



As instances of adaptation to a peculiar and phylogenetically 

 unnatural environment I may mention the flying mammals (bats); 

 the pelagic mammals (cetaceans) ; the terrestrial birds (kiwi, ostrich, 

 etc.); the pelagic birds (penguins); the flying reptiles (pterodactyls); 

 the pelagic reptiles (sea snakes, plesiosauri, ichthyosauri, etc.) ; the 

 purely aquatic amphibians (Amphiuma, Siredon, Triton, Siren, etc.); 

 the purely terrestrial amphibians (Hylodes, etc.) ; the semiterres trial 

 fishes; the flying fishes; the parasitic or symbiotic fishes; the purely 

 pelagic fishes; the deep-sea fishes; the parasitic insects; the aquatic 

 insects; the aquatic arachnids; the parasitic arachnids; the mud- 

 inhabiting holothurians ; the deep-sea holothurians ; the pelagic holo- 

 thurians; the mud-living echinoids ; the sessile gastropods; the para- 

 sitic gastropods ; the parasitic "worms;" the sessile ctenophores; the 

 parasitic barnacles; the free-living barnacles; the parasitic Crustacea; 

 the pelagic Crustacea; the deep-sea Crustacea. A large number of 

 additional instances could be brought forward, but a study of the 

 types mentioned is sufficient to show at once that when an animal 

 acquires a phylogenetically new environment a readjustment of its 

 structure is induced which often leads to very remarkable changes, 

 so that the real affinities of the animal are rendered very difficult of 

 appreciation. 



Among the echinoderms a few cases seem worthy of special atten- 

 tion. The elasipod holothurians, which live in deep water, are bilat- 

 erally symmetrical, with tube feet on the ventral side only, and with 

 papillae on the back; there are no respiratory trees or Cuvierian organs. 

 The apodal holothurians, which commonly live in mud, have no 

 radial canals, tube feet, or respiratory trees. The free-swimming 

 pelagic holothurians have no calcareous deposits whatever, no respira- 

 tory trees, no Cuvierian organs, no retractor muscles, and single Ion- 



